Xcel Energy got a green light from the state legislature Tuesday to start a comprehensive plan to cut air pollution at aging coal-fired power plants — with “primary consideration” to switching to natural gas.

On a voice vote, the state Senate approved a bill that requires Xcel Energy to cut emissions of nitrogen oxides by 70 to 80 percent by 2017 at aging Front Range coal plants.

The state has to file cleanup plans for each with the Environmental Protection Agency starting next January.

“This is clearly a smart way to do it,” said Colorado Public Utilities Commission chairman Ron Binz. “I don’t think any other state has taken this comprehensive approach.”

The bill — negotiated among natural gas representatives, Xcel executives and environmental groups — is supported by Gov. Bill Ritter. House Bill 1365 was introduced in the legislature 16 days ago and has been passed by the House. After a third reading in the Senate, it will go to Ritter for his signature.

Supporters of the bipartisan bill, led by Senate Minority Leader Josh Penry, R-Grand Junction, had to turn back a volley of amendments mostly from Republican lawmakers.

The bill, Penry said, offers a chance for Colorado to get out in front of the federal clean-air mandates. “If we don’t do it, the EPA will,” Penry warned.

A key to the plan is using more natural gas — “a clean-burning fuel we have a heck of a lot of here in Colorado,” Penry said.

Still, Senate Republicans, especially those with coal mines and coal-fired power plants in their districts, opposed the bill.

The bill will “shoot the coal industry in the head,” said Sen. Al White, R-Hayden, adding that two Xcel plants switching to natural gas could cost 125 mining jobs in his region.

Penry and Bruce Whitehead, D-Hesperus, the other Senate co-sponsor, countered that the bill will create jobs.

“It is not coal versus gas. It is about our children’s future,” Whitehead said. “It is about cleaning the air.”

Natural-gas industry executives testifying at a Senate Agriculture, Livestock and Natural Resources Committee hearing last week estimated that the bill could create 400 new jobs.

An amendment by Sen. Shawn Mitchell, R-Broomfield, to remove references to natural gas and substitute low-emitting fuel also was opposed by the sponsors.

“Let natural resources compete to satisfy our needs,” Mitchell said.

Some lawmakers also tried to remove a section enabling Xcel to add construction costs for the project as they are incurred rather than waiting until the work is completed.

The incentives are needed because utilities have been “skewed toward capital-intensive coal plants,” which are more easily included in rates, said Assistant Minority Leader Greg Brophy, R-Wray.

“It puts gas on a more level playing field,” Penry said.

Xcel will be required to submit a plan to the PUC in August to close, retrofit with coal-pollution equipment or switch to gas at some of its smaller Front Range plants.

“We will analyze and model all the assumptions about both fuels,” the PUC’s Binz said.

Another incentive in the bill is the guarantee that long-term contracts between natural-gas producers and utilities to do away with big swings in gas prices will be honored by the PUC, Binz said.

The bill directs Xcel to come up with a plan for reduced emissions on 900 megawatts of generation. The Valmont Station in Boulder and the Cherokee plant in Commerce City equal 900 megawatts. Both were designed to use coal or natural gas and are more than 40 years old.

“The clean-air bill is a significant step in addressing our brown cloud in the winter, smog in the summer and lifting the haze that causes a pall over our national parks,” said Vickie Patton of the Environmental Defense Fund.

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